The present application relates to a technique of using electromagnetic waves in a band of about 0.1×1012 THz to 100×1012 THz.
A known technique of generating or detecting terahertz waves is terahertz time-domain spectroscopy (THz-TDS). As known in the art, the terahertz time-domain spectroscopy is suitable for use in imaging samples because the THz-TDS utilizes terahertz waves that define ultra-short pulses, as short as about 100 femtoseconds. Therefore, the terahertz time-domain spectroscopy attracts attention in various technical fields such as industry, medical care, biotechnology, agriculture and security.
In the terahertz time-domain spectroscopy, a pulse light beam emitted from an ultra-short laser source is split into a pump beam and a probe beam. The pump beam is focused on a terahertz-wave generating element. In the terahertz-wave generating element, a current flow or electrical polarization lasts for about subpico seconds, generating a terahertz wave having field amplitude proportional to the time derivative. The terahertz wave is focused by an optical system on a terahertz-wave detecting element. At this point, the probe beam is applied to the terahertz-wave detecting element. Then, the terahertz-wave detecting element generates a carrier. The carrier is accelerated by the electrical field of the terahertz wave and changed to an electric signal. The time at which the probe beam reaches the terahertz-wave detecting element is delayed, thereby measuring the time waveform the terahertz wave has in the amplitude electric field. The time waveform is Fourier-transformed, thereby determining the spectrum of the terahertz wave.
In an apparatus that performs the terahertz time-domain spectroscopy, an optical system guides, to the sample, the terahertz wave generated in the terahertz-wave generating element, and guides, to the terahertz-wave detecting element, the terahertz wave generated in the sample. In most cases, as shown in FIG. 1, this optical system has four parabolic mirrors PM1 to PM4.
This optical system is a transmission type that detects the terahertz wave that has passed though the sample. A reflection-type optical system is available, which detects the terahertz wave reflected by the sample (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2004-191302). The parabolic mirrors may be replaced by small-aperture optical lenses, rendering the lens system small (see, for example, PCT National Publication No. 2003-518617).